The invention relates to a configuration for the low-loss writing of an Magneto-Resistive Random Access Memory (MRAM) having a plurality of cells that are provided between respective word lines and bit lines in a cell field, whereby, given writing into a particular memory cell, a voltage drop occurs on the selected word line and bit line that are connected to such a cell.
FIG. 4A is a schematic illustration of a conventional MRAM cell with a word line WL running in direction y and a bit line BL running in the perpendicular direction x, which crosses the word line at a distance. Between the word line WL and the bit line BL, there is a conventional memory cell Z, shown in FIG. 4B, which includes a hard-magnetic layer 1, a tunnel barrier layer 2, and a soft-magnetic layer 3 disposed in a layer stack between the word line WL and the bit line BL.
To store the desired data item in such an MRAM cell, a current IWL is impressed on the word line WL, and a current IBL is impressed on the bit line BL. These currents IWL and IBL generate magnetic fields BWL and BBL, respectively. Thus, at the crossing of the word line WL and bit line BL, i.e., in the region of the memory cell Z, a magnetic field BxWL runs in direction x due to the current IWL flowing through the bit line BL, and a magnetic field ByBL runs in direction y due to the current IBL flowing through the bit line BL. The total magnetic field B formed by the sum of the two magnetic fields BxWL and ByBL directs the soft-magnetic layer 3 of the cell Z in a particular direction, which may be parallel or antiparallel to the magnetization of the hard-magnetic layer 1. The memory cell Z thus stores a logical 1 or 0 depending upon the parallel or antiparallel magnetization of the two layers 1 and 3, to which a low or high resistance value is allocated, respectively.
In a write operation, the word line current IWL flows through the word line WL. But because the word line WL has a resistance RL in each of its subsections between individual bit lines BL0, BL, . . . , a voltage drop UL occurs in each subsection as a result of the line resistance along the word line WL. Such voltage drop UL brings about voltage differences UZ0, UZ1, UZ2, . . . across the individual cells Z, thereby causing parasitic currents Ipar0, Ipar1, Ipar2, . . . to flow through the cells Z, as represented in FIG. 5.
The current IWL flowing in the word line WL is damped along the word line WL by these parasitic currents Ipar0, Ipar1, . . . , and, therefore, reliable writing, which requires a certain intensity of the current IWL in the word line WL, can no longer be guaranteed. In other words, due to the parasitic currents Ipar0, Ipar1, . . , the strength of the current IWL flowing in the word line WL must be elevated.
But such an elevation of the current IWL flowing in the word line WL is limited, because an excessive current IWL in the word line WL could lead to the writing of all cells along such a word line WL without the co-operation of a respective bit line BL0, BL1. In other words, given an excessively high current in the word line WL, it is no longer possible to select the memory cells.
To keep the parasitic currents Ipar0, Ipar1, . . . optimally small given these conditions, it would be imaginable either to provide for an optimally high resistance of the memory cells or to shorten the length of the word lines WL. However, both measures are associated with material disadvantages in that high resistances of the memory cells reduce the read current through these cells and, thus, complicate a reliable reading. On the other hand, short word lines reduce the efficiency of the MRAM, that is to say, of the memory chip, and, thus, raise the cost of production. The same concerns arise with respect to the bit lines, accordingly.
It is accordingly an object of the invention to provide a configuration for the low-loss writing of an MRAM that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type and that provides a configuration for the low-loss writing of an MRAM that utilizes neither high cell resistances nor short word lines and/or bit lines.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a configuration for low-loss writing of a magneto-resistive random access memory, including a cell field having word lines, bit lines, and memory cells disposed in the cell field between respective ones of the word lines and the bit lines. The memory cells are configured to have a voltage drop occur on a selected one of the word lines connected to a particular one of the memory cells when writing into the particular one of the memory cells and to have voltages at the bit lines set to minimize a cell voltage across the memory cells between a selected one of the word lines and individual ones of the bit lines.
In the configuration according to the invention, the voltages at the bit lines and word lines are set such that the cell voltage across the memory cells between the selected word line and the individual bit lines, and between the selected bit line and individual word lines, is minimal.
In the configuration, the cell voltage that arises in the write operation and the parasitic currents flowing through the individual cells are reduced or even eliminated by suitably adjusting or regulating the voltages at the individual bit lines and word lines.
With the objects of the invention in view, there is also provided a configuration for the low-loss writing of a magneto-resistive random access memory, including a cell field having word lines, bit lines, and memory cells disposed in the cell field between respective ones of the word lines and the bit lines. The memory cells are configured to have a voltage drop occur on a selected one of the word lines connected to a particular one of the memory cells when writing into the particular one of the memory cells, the selected word line having two ends, and when a voltage V1 and a voltage V2 less than V1 are present at a respective one of the two ends of the selected word line, the cell field is configured to have all of the bit lines set to voltages (V1+V2)/2 and to have a maximum cell voltage of xc2x1(V1xe2x88x92V2)/2.
To accomplish the result, there are principally two variants, which will be described in detail below with reference to a selected word line. The relations that are described for the selected word line also substantially apply to a selected bit line. A combination of a selected word line and a selected bit line is also possible: whereas all bit lines are controlled to a voltage of the selected word line, at the same time all word lines are controlled to the voltage of the selected bit line.
(a) When the voltage drop along a word line has the magnitude V1xe2x88x92V2, with V1 being the voltage at one end of the word line and V2 being the voltage at the other end, then all bit lines are set to the voltage (V1xe2x88x92V2)/2. A maximum cell voltage of xc2x1(V1xe2x88x92V2)/2 is then present. Accordingly, the parasitic currents that flow in across the memory cells on one half of the word line flow out again on the other half of the word line. In other words, in the variant, the bit lines are all set to a separate equipotential precisely midway between U1 and U2.
(b) In a second variant, in contrast to variant (a), the voltages of the individual bit lines are not set to an equipotential. Rather, they are individually conditioned to the voltage drop along the word line so that the voltage across the individual memory cells is approximately zero, and practically no parasitic current will flow. Because the measuring of the word line voltage at each individual cell along the selected word line to recover reference voltages demands a large chip area, a reference word line is expediently inserted, which simulates the selected word line and from which the reference voltage is drawn, which is applied to the respective bit line by buffers or isolation amplifiers. To achieve additional savings of chip space, several bit lines can be combined into a group and set to an equipotential corresponding to the mean value of the voltage in a part of the word line that is respectively allocated to the group of bit lines.
In accordance with another feature of the invention, the bit lines are set to a same potential as the selected word line in the parts allocated to the individual bit lines.
In accordance with an added feature of the invention, several of the bit lines are combined into a group, the selected word line has a part allocated to the group, the part having a voltage impressed thereon, and the several bit lines are set to an equipotential corresponding to a mean value of the voltage in the part.
With the objects of the invention in view, there is also provided a configuration for the low-loss writing of an magneto-resistive random access memory, including a cell field having word lines, bit lines, and memory cells disposed in the cell field between respective ones of the word lines and the bit lines. The memory cells are configured to have a voltage drop occur on selected ones of the bit lines connected to a particular one of the memory cells when writing into the particular one of the memory cells and to have voltages at the word lines set to minimize a cell voltage across the memory cells between a selected one of the bit lines and individual ones of the word lines.
With the objects of the invention in view, there is also provided a configuration for the low-loss writing of a magneto-resistive random access memory, including a cell field having word lines, bit lines, and memory cells disposed in the cell field between respective ones of the word lines and the bit lines. The bit lines are configured to have a voltage drop occur on selected ones of the bit lines connected to a particular one of the memory cells when writing into the particular one of the memory cells, the selected bit lines each having two ends, and, when a voltage V1 and a voltage V2 less than V1 are present at a respective one of the two ends of the selected bit lines, the cell field is configured to have all of the word lines set to voltages (V1+V2)/2 and to have a maximum cell voltage of xc2x1(V1xe2x88x92V2)/2.
In accordance with an additional feature of the invention, the selected ones of the bit lines have parts, and the word lines are set to a same potential as the selected one of the bit lines in the parts allocated to the individual ones of the word lines.
In accordance with yet another feature of the invention, the cell field has a reference bit line simulating the selected one of the bit lines.
In accordance with yet a further feature of the invention, several of the word lines are combined into a group, the selected bit line has a part allocated to the group, the part having a voltage impressed thereon, and the several word lines are set to an equipotential corresponding to a mean value of the voltage in the part.
In accordance with yet an added feature of the invention, several of the word lines are combined into a group, the selected bit line has a part allocated to the group, the part having a voltage impressed thereon, and the several word lines are set to an equipotential corresponding to a mean value of the voltage in the part.
With the objects of the invention in view, there is also provided a method for the low-loss writing of a magneto-resistive random access memory, including the steps of providing memory cells in a cell field between respective word lines and bit lines, creating a voltage drop on a selected one of the word lines connected to a particular one of the memory cells when writing into the particular memory cell, and setting voltages at the bit lines to minimize a cell voltage across the memory cells between the selected word line and each individual one of the bit lines.
With the objects of the invention in view, there is also provided a configuration for the low-loss writing of a magneto-resistive random access memory, including the steps of providing memory cells in a cell field between respective word lines and bit lines, creating a voltage drop on a word line connected to a particular one of the memory cells when writing into the particular memory cell, and setting, when a voltage V1 and a voltage V2 less than V1 are present at two ends of a selected word line, all bit lines to voltages (V1+V2)/2 such that a maximum cell voltage is xc2x1(V1xe2x88x92V2)/2.
In accordance with yet an additional mode of the invention, the bit lines are set to a same potential as a respective word line in respective parts of the word line allocated to individual bit lines.
In accordance with again another mode of the invention, the selected word line is simulated with a reference word line.
In accordance with again a further mode of the invention, several bit lines are combined into a group, and the group of bit lines are set to an equipotential corresponding to a mean value of a voltage in a part of a word line allocated to the group.
With the objects of the invention in view, there is also provided a method for the low-loss writing of a magneto-resistive random access memory, including the steps of providing memory cells in a cell field between respective word lines and bit lines, creating a voltage drop on selected bit lines connected to a particular one of the memory cells when writing into the particular memory cell, and setting voltages at the word lines to minimize a cell voltage across the memory cells between the selected bit lines and each individual one of the word lines.
With the objects of the invention in view, there is also provided a method for the low-loss writing of a magneto-resistive random access memory, including the steps of providing memory cells in a cell field between respective word lines and bit lines, creating a voltage drop on selected bit lines connected to a particular one of the memory cells when writing into the particular memory cell, and setting, when a voltage V1 and a voltage V2 less than V1 are present at two ends of a selected bit line, all word lines to voltages (V1+V2)/2 such that a maximum cell voltage is (V1xe2x88x92V2)/2.
In accordance with again an added mode of the invention, the word lines are set to the same potential as a respective bit line in respective parts of the bit line allocated to individual word lines.
In accordance with again an additional mode of the invention, the selected bit line is simulated with a reference bit line.
In accordance with a concomitant mode of the invention, several word lines are combined into a group, and the group of word lines is set to an equipotential corresponding to a mean value of a voltage in a part of a bit line allocated to the group.
Other features that are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a configuration for the low-loss writing of an MRAM, it is, nevertheless, not intended to be limited to the details shown because various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.